Polymeric Curing agent modified with hexamethyldisilazane (PCA-D), or with hexamethylcyclotrisilazane (PCA-T), was used to improve the mechanical properties of hydroxyl-teminated polydimethylsiloxane (PDMS) rubber. The structure and the gel time of PCA were characterized by Si-29 NMR and shear viscosity measurement, respectively. The PCA modified with silazanes was more stable in storage than that without treatment (PCA-0). Chemical bonds were formed during the reaction of silazanes and PCA according to Si-29 NMR results. The crosslink density (gamma(e)) and the mechanical properties of PCA/PDMS rubber were determined by swelling equilibrium and stress-strain tests. It was found that PCA treated with both silazanes could better enhance the mechanical properties of PCA/PDMS rubber compared with PCA-0. PCA-T/PDMS rubber, with additional crosslinks, was the best among the three types of PCA/PDMS rubber on the mechanical properties. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1057-1062, 2009